Various types of magnetic field sensing elements are known, including Hall effect elements and magnetoresistance elements. Magnetic field sensors generally include a magnetic field sensing element and other electronic components. Some magnetic field sensors also include a fixed permanent magnet.
Magnetic field sensors provide an electrical signal representative of a sensed magnetic field, or, in some embodiments, fluctuations of the magnetic field associated with the magnet. In the presence of a moving ferromagnetic object, the magnetic field signal sensed by the magnetic field sensor varies in accordance with a shape or profile of the moving ferromagnetic object.
Magnetic field sensors are often used to detect movement of features of a ferromagnetic gear, such as gear teeth and/or gear slots. A magnetic field sensor in this application is commonly referred to as a “gear tooth” sensor.
In some arrangements, the gear is placed upon a target object, for example, a camshaft in an engine, thus it is the rotation of the target object (e.g., camshaft) that is sensed by detection of the moving features of the gear. Gear tooth sensors are used, for example, in automotive applications to provide information to an engine control processor for ignition timing control, fuel management, and other operations.
Information provided by the gear tooth sensor to the engine control processor can include, but is not limited to, an absolute angle of rotation of a target object (e.g., a camshaft) as it rotates, and a direction of rotation. With this information, the engine control processor can adjust the timing of firing of the ignition system and the timing of fuel injection by the fuel injection system.
Many types of magnetic field sensors do not provide an accurate output signal (e.g., indication of absolute angle of rotation of an object) immediately upon movement of the target object from zero rotating speed and/or upon movement slowing to zero rotating speed, but instead provide an accurate output signal only once the target object has moved through a substantial rotation or is moving with substantial speed. For example, in one type of magnetic field sensor described in U.S. Pat. No. 6,525,531, issued Feb. 25, 2003, a positive digital-to-analog converter (PDAC) and a negative digital-to-analog converter (NDAC) track positive and negative peaks of magnetic field signal, respectively, for use in generating a threshold signal. A varying magnetic field signal is compared to the threshold signal. However, the outputs of the PDAC and the NDAC may not be accurate indications of the positive and negative peaks of the magnetic field signal until several cycles of the signal (i.e., signal peaks) occur (i.e., until several gear teeth have passed). This type of magnetic field sensor, in which the threshold signal is a function of the positive and negative peaks of the magnetic field signal, is referred to herein as a so-called “running mode detector.”
In contrast, a “true power on state” (TPOS) detector can provide an accurate output signal (e.g., indication of absolute angle of rotation) shortly after movement of a target object (e.g., camshaft) from zero rotating speed or also shortly before movement slowing to zero rotating speed. Furthermore, even when the target object is not moving, the TPOS detector can provide an indication of whether the TPOS detector is in front of a tooth or a valley. However, when the target object is stationary, the conventional TPOS detector is not able to identify an absolute or relative angle of rotation of the target object. The TPOS detector can be used in conjunction with a running mode detector, both providing information to the engine control processor.
As described above, the conventional TPOS detector provides an accurate output signal with only a small initial rotation of the target object, and before the running mode detector can provide an accurate output signal. The TPOS detector can provide information to the engine control processor that can be more accurate than information provided by the running mode detector for time periods at the beginning and at the end of rotation of the target object (e.g., start and stop of the engine and camshaft), but which may be less accurate when the object is rotating at speed. When the object is rotating at speed, the engine control processor can primarily use rotation information provided by the running mode detector. In most conventional applications, once the magnetic field sensor switches to use the running mode detector, it does not return to use the TPOS detector until the target object stops rotating or nearly stops rotating.
A conventional TPOS detector is described in U.S. Pat. No. 7,362,094, issued Apr. 22, 2008. The conventional TPOS detector includes a comparator for comparing the magnetic field signal to a fixed, often trimmed, threshold signal. The conventional TPOS detector can be used in conjunction with and can detect rotational information about a TPOS cam (like a gear), which is disposed upon a target object, e.g., an engine camshaft, configured to rotate.
An output signal from a conventional TPOS detector has at least two states, and typically a high and a low state. The state of the conventional TPOS output signal is high at some times and low at other times as the target object rotates, in accordance with features on the TPOS cam attached to the target object. An output signal from a conventional TPOS detector is shown and described below in conjunction with FIG. 2.
Though the TPOS detector provides rotation information (e.g., angle of rotation) faster than the running mode detector once a target object begins to rotate, it does not necessarily provide the rotation information immediately at the start of the rotation.
Thus, it would be desirable to provide a magnetic field sensor and technique that can generate a signal representative of at least the absolute angle of rotation of a target object, either continuously, e.g., when the target object is not rotating, or, at least when the target object rotates through only a very small number of degrees, smaller than for a conventional TPOS detector. It would also be desirable to provide a magnetic field sensor and technique for which the output signal is the same as or similar to the output signal of a known TPOS detector. Having characteristics of a known TPOS detector, the magnetic field sensor could be used in an engine control system without modification to the conventional engine control processor or software code used by the engine control processor.